The present invention relates to an ink jet recording sheet that exhibits good ink fixing performance while spread of ink dots especially under a high humidity circumstance is prevented and further fading or change in color of the recorded images that is apt to occur with lapse of time is inhibited.
The ink jet performs recording of images or letters by depositing ink droplets ejected by various working principles on a recording sheet such as a paper. The ink jet recording has such favorable features that it makes high-speed recording possible, that it produces little noise, that it can easily perform multi-color recording, that there is no limitation as to kind of patterns or images, and that it requires no processing for development and fixing. Thus, the ink jet recording is rapidly becoming widespread in various fields as devices for recording various images including kanjis (Chinese characters) and color images. Furthermore, the images formed by the multi-color ink jet recording method are by no means inferior to those printed by a multi-color press or those obtained by a color-photography. Besides, use of the ink jet recording extends to a field of full-color image recording where number of copies is not so many, since costs per copy are less expensive than those employing the photographic processes.
As for the recording sheets for ink jet recording, efforts have been made from the aspects of printer hardwares or ink compositions in order to make use of woodfree papers or coated papers used for ordinary printing or writing. However, improvements in recording sheets have come to be required increasingly in order to go side by side with developments in printer hardwares such as ever increasing speed, development of ever finer definition image of full color, and also with expanding fields of uses. That is, recording sheets are demanded to develop ever high reproducibility image, and in order to meet that demand image density of the printed dots be maintained high, hue characteristics be bright and appealing, the ink applied be fixed quickly and does not bleed or spread even though a different color ink is put over additionally. Moreover, ink should set quickly, dots should not spread more than needed and the circumference of the dots be sharp and demarcating.
Some proposals have, hitherto, been made for meeting these demands. For example, it is attempted to improve ink absorbing property by providing an ink-receiving layer mainly composed of a silica pigment thereby letting it serve an absorbing layer on the surface of a support Japanese Patent Kokai (Laid-Open) Nos. 52-9074 and 58-72495!. In order to improve the ink absorbing property of this ink-receiving layer and to obtain a high image density of printed dots while keeping their spread in restraint, Japanese Patent Kokai (Laid-Open) Nos. 55-51583 and 56-157 propose to add non-colloidal silica powders to the ink-receiving layer. A further attempt looks at the dye distribution state in the ink-receiving layer as a factor influencing tinctorial characteristics and sharpness, and proposes to use a specific dye-absorbing agent which adsorbs the dye component in the ink Japanese Patent Kokai (Laid-Open) No. 55-144172!.
The above proposals are successful in keeping dots spread in restraint and in developing images excellent in sharpness and color quality, but only under relatively favorable environment. Those successful effects can hardly be maintained always when the recording and storing environment changes. Especially, when recording is carried out or the printed record is stored under a high humidity condition, the ink that is usually aqueous and has set once becomes liquid by absorbing moisture and comes to bleed. Another problem is that the colorant changes with time to cause discoloration of the recorded image. The aforesaid bleeding of ink leads to enlargement of dots diameter bringing about degradation in sharpness, color quality, and image reproducibility. In addition, discoloration of the recorded image results in not only the degradation of color quality, but also the problem of forming images of utterly different color.
If these problems happen, value as an ink jet recording sheet is impaired, and the following measures can be considered for avoiding the problems. For developing sharpness of the image, it is preferred that horizontal spread of an ink dot is restrained; for maintaining the color quality of the images, it is preferred that the colorant of the ink remains on the surface of the recording sheet. Thus, it is ideal that only the vehicle of the ink permeates into inside of the recording sheet leaving colorant at its surface. On the other hand, for avoiding discoloration with lapse of time, the colorant preferably penetrates with the vehicle into a depth from the surface of the recording sheet so that it may be protected against external factors like light or ozone gas. Thus, the restraint of dots spread and prevention of color quality degradation are conflicting with each other and it is difficult to solve both of them simultaneously.
With the recent proliferation of ink jet printers, the environment in which printing is carried out by ink jet recording is diversified considerably, and such image reproducibility as not affected by environment is demanded. Furthermore, in the field of full-color hard copy in which the ink jet recording method has now come to be employed increasingly, storage stability is demanded ever eagerly. Thus, ink jet recording sheets satisfying these demands are needed.